The blocks which are set on the anvil resemble these stock types. In many cases they are used on anvil, or bending block, or stand Another group which is emindifferently. ployed either on anvil, or bending block, is of a special character, for work bent in a vertical plane, and secured with cottars, the block being made for the jobs, and having shapes the exact counterparts of the latter. The cottars pass through straps bolted to the sides of the bending block.

The general ways of making bending blocks are as follows:-A pattern is prepared in wood to the section and curvature required, and moulded from, holes being cast in, by which it is bolted to the main bending block. The small blocks to be used either on the anvil, or on the large bending block have a forged steel stud cast on the bottom to fit one of the holes. Some light blocks are built up of bars or rods, but the majority are cast as just stated. A block may be of such a size to comprise the entire area or surface against which the work is to be bent, as for example a complete ring or disc, or it may in the case of large work only include a segmental portion of the same. In the first case the work would be bent at one operation, and frequently at one heat. In the second it would be done in successive stages, or sections, and at more than one heat.

Bending Machines for Plates.Heavy plates are preferably bent in a machine which squeezes in successive sections at intervals of about 3 inches. In this, vertical dies, Fig. 149, are used. One die or girder is fixed, the other is movable and is forced against the plate by means of rams and toggle levers. The photo, Fig. 150, Plate IV., shows a type by Fielding & Platt, Ltd., in which the movement is obtained from the ram by means of inclined planes.

Bending Moment.-Is composed of force multiplied by distance, acting upon a section and tending to cause its failure by crushing the fibres in one portion and tearing them apart in another.

If we take Fig. 88 on p. 110 the maximum

There are two broad types of bending rolls, the horizontal, and the vertical, the latter being employed chiefly for very heavy work. This has the advantage of affording facilities for the better handling and movement of plates, which

can be slid along edgewise, supported on rollers, and which if very long, do not curve up or down awkwardly, as when bent in horizontal rolls.

The essential action of the common rolls is seen in Fig. 151, A, applicable to horizontal and vertical types, since there is no difference in their principle of operation. There are three rolls, the centres of two being fixed, those of the other adjustable in a plane perpendicular to the plane joining the centres of the fixed ones. By this adjustment curves of different radii are produced, or combinations of curves, by varying the centres during the passage of a plate, the attendant judging of the amount as the work proceeds. The principle is therefore simple. But the practice involves much variation in detail. Around these three rolls the machines built number many designs, see Figs. 152157, Plates IV.-VII.; light and heavy, with various methods of driving, by belt and gears, by independent engines, and by motors, with different kinds of framings, and adjusting mechanisms, with provision for getting out complete rings, with rolls made of different materials, and supported, or unsupported below. The elasticity of the rolls, evident when rolling thick plates, is the reason for the latter provision.

A fault of all these rolls is that they cannot produce a perfect circle, or circular arc right to the edges, but must leave flats at both entering and leaving edges, due to the separation of the bottom rolls.